A research team from KAIST's School of Electrical Engineering has developed an ultralow-power carbon dioxide (CO2) sensor using a flexible and thin organic photodiode. By attaching the sensor to a standard face mask, the team successfully demonstrated real-time breathing monitoring.
From the left, School of Electrical Engineering, Ph.D. candidate DongHo Choi, Professor Seunghyup Yoo, and Department of Materials Science and Engineering, Bachelor’s candidate MinJae Kim. Image Credit: KAIST
With low power consumption, high stability, and flexibility, wearable devices like this could play a key role in the early diagnosis of respiratory conditions such as chronic obstructive pulmonary disease (COPD) and sleep apnea.
CO2 is a major respiratory metabolite, and its continuous monitoring provides crucial insights into respiratory and circulatory system diseases. It can also be used to assess exercise performance. The KAIST team successfully measured CO2 concentrations with precision by embedding the sensor inside a mask.
KAIST recently announced that Professor Yoo and his team had developed a compact, high-speed wearable CO2 sensor designed for stable, real-time breathing analysis.
Existing non-invasive CO2 sensors are typically bulky and power-hungry. Optochemical CO2 sensors, which rely on fluorescent molecules, offer advantages in miniaturization and weight but suffer from photodegradation, limiting long-term usability in wearable healthcare applications.
These sensors detect CO2 concentration by measuring changes in fluorescence intensity. The research team refined the detection process to enhance accuracy and efficiency.
To achieve this, they developed a CO2 sensor with an LED and an organic photodiode, optimizing light collection to minimize excitation light exposure on fluorescent molecules. This breakthrough slashed the sensor’s power consumption to just 171 μW—significantly lower than conventional models consuming several milliwatts.
Further, the team identified the photodegradation pathway of fluorescent molecules in CO2 sensors, pinpointed error causes over time, and proposed an optical design to counteract these issues.
The upgraded sensor significantly extends the operational time, allowing continuous use for up to 9 hours—far surpassing the under-20-minute lifespan of existing technologies. It can also be reused by replacing the CO2 detection fluorescent film. Weighing just 0.12 grams and measuring only 0.7 mm in thickness, the sensor is lightweight, ultra-thin, and flexible. It offers high resolution and rapid response times, accurately distinguishing between inhalation and exhalation in real-time.
The developed sensor has excellent characteristics such as low power, high stability, and flexibility, so it can be widely applied to wearable devices, and can be used for the early diagnosis of various diseases such as hypercapnia, chronic obstructive pulmonary disease, and sleep apnea.
Seunghyup Yoo, Professor, Department of Electrical and Electronic Engineering, Korea Advanced Institute of Science & Technology (KAIST)
Yoo added, “In particular, it is expected to be used to improve side effects caused by rebreathing in environments where dust is generated or where masks are worn for long periods of time, such as during seasonal changes.”
Journal Reference:
Kim, M., et al. (2025) Ultralow-power carbon dioxide sensor for real-time breath monitoring. Device. doi.org/10.1016/j.device.2024.100681